Working with an in vitro model for invasion of red cells by malaria merozoite, this study has demonstrated that human red cells lacking the Duffy a and b antigens are resistant to invasion. This corresponds well with epidemiologic studies which show that approximately 70% of West African and American blacks are resistant to Vivax malaria. A similar proportion of these blacks are Duffy a and b negative whereas the Duffy negative genotype is extremely rare in other racial groups. The close association between the Duffy negative genotype and resistance to Vivax malaria has also been confirmed by retrospective analysis of 11 volunteers experimentally inoculated with P Vivax. A larger study in humans is planned.